When it comes to breaking a line of grid item like this, the image shown below illustrates how the rest space of the grid should be empty.
https://i.stack.imgur.com/pvSwo.png<Grid container spacing={3}
<Grid item xs={12}>
"Grid Item xs={12}"
</Grid>
<Grid item xs={4}>
"Grid Item xs={4}"
</Grid>
// empty space
<Grid item xs={12}>
"Grid Item xs={12}"
</Grid>
</Grid>
Are you wondering whether to use an inner grid container
or opt for display flex
instead?